Ceramic materials have enjoyed great success as igniters in gas fired furnaces, stoves and clothes dryers. A ceramic igniter typically has a hairpin or U-shape which contains conductive end portions and a highly resistive middle portion. When the igniter ends are connected to electrified leads, the highly resistive middle portion (or "hot zone") rises in temperature.
The art of ceramic igniters has long known of hairpin-shaped igniters which further have an electrically non-conductive ceramic insert disposed between their electrically resistive legs for support. JP-A-02094282 specifically discloses a ceramic igniter having SiC/ZrB.sub.2 resistive legs and an AlN insulating insert (or "support zone")disposed between the resistive legs. JP-A-02094282 further teaches adding BN to the AlN insert in order to match the coefficients of thermal expansion ("CTE") of the two regions. Similarly, U.S. Pat. No. 5,191,508 ("Axelson")discloses a hairpin-shaped ceramic igniter having an "electrically non-conductive" insert, and teaches that the insert should be made from a single material such as alumina, aluminum nitride, beryllium oxide, each of which are electrically insulating materials. U.S. Pat. No. 4,634,837 ("Ito") discloses a ceramic igniter having a Si.sub.3 N.sub.4 /MoSi.sub.2 -based hot zone and a Si.sub.3 N.sub.4 /Al.sub.2 O.sub.3 insert.
The art also discloses ceramic igniters in which conductive filaments are embedded in insulative ceramic materials. For example, U.S. Pat. No. 4,912,305 ("Tatemasu") discloses a tungsten wire embedded in a Si.sub.3 N.sub.4 /Al.sub.2 O.sub.3 /Y.sub.2 O.sub.3 ceramic body. U.S. Pat. No. 4,804,823 ("Okuda") discloses a ceramic igniter in which a TiN or WC conductive ceramic layer (which also contains Si.sub.3 N.sub.4) is disposed within a ceramic substrate of either AlN or Si.sub.3 N.sub.4. Okuda also discloses that the substrate may further contain a sintering aid such as an oxide, nitride, or oxynitride of Groups Iia or IIIa of the Periodic Table or Aluminum. See column 7 lines 50-55.
Although the insert material in hairpin shaped igniters is generally highly electrically insulating, there are instances in which the art has disclosed inserts having some electrically conductive (such as MoSi.sub.2) and/or semiconductive components (such as SiC). For example, JP-A-02086 ("JP '086") provides one such disclosure wherein the main constituent of the insert is silicon carbide. However, research has shown that the high temperature resistivities of a first material comprising SiC and a conductive material such as aluminum and a second material comprising over 99% SiC tend to equalize at high temperatures. Therefore, if these materials were to be used respectively as a hot zone and an insert in the same igniter, there would likely be electrical shorts across the insert material. In another example, U.S. Pat. No. 5,233,166 ("Maeda") discloses an igniter having a hot zone embedded in a ceramic substrate comprising silicon nitride, 8-19% rare earth oxide, 2-7% silica, and 7-20% MoSi.sub.2. Maeda teaches to avoid producing a glass phase having alumina in an amount of more than 1 wt %.
U.S. Pat. No. 5,801,361 (Willkens '361) discloses a ceramic igniter designed for use in high voltage applications (220 V-240 V) in which the conventional hairpin-shaped hot zone is supported by ceramic material both between its legs and outside of its legs by support zones. Willkens '361 also teaches that this support zone material should be electrically insulating (i.e., should have an electrical resistivity of at least 10.sup.6 ohm-cm) and should preferably comprise at least 90 vol % of at least one of aluminum nitride, boron nitride and silicon nitride. Willkens '361 further discloses that this support zone material should not only possess thermal expansion and densification characteristics which are compatible with the hot zone, but also help protect the hot zone from oxidation (i.e., less than 10% amperage decrease over 30,000 cycles). In a WIPO publication corresponding to Willkens '361, the suggested electrical resistivity of the support zone material is 10.sub.8 ohm-cm.
However, although the igniter of Willkens '361 attains the required performance specifications for voltage applications, continued use of the igniter revealed significant long-term use failures in one support zone consisting essentially of aluminum nitride (AlN). That is, the resistance of this igniter increased significantly during extended use trials. Furthermore, densification problems (likely due to thermal expansion mismatch) were encountered with these support zones during manufacture. Lastly, Willkens '361 observed that, in one example, the white-hot glow of the hot zone (which had a room temperature resistivity of about 0.3 ohm-cm) tended to creep downwards, and suggested that this creep was caused by current flowing through the aluminum nitride-based insert.
U.S. Pat. No. 5,786,565 (Willkens '565) discloses another ceramic igniter having a support zone (or "insert")disposed between the two parallel legs of the igniter. According to Willkens '565, this insert is referred to as an "electrically insulating heat sink" or as an "electrically non-conducting heat sink", preferably has a resistivity of at least about 10.sup.4 ohm-cm. Preferably, the composition of the insert comprises at least 90 vol % of at least one of aluminum nitride, boron nitride and silicon nitride, but more preferably it consists essentially of at least one of aluminum nitride, boron nitride and silicon nitride.
However, although the igniters of Willkens '565 were found to possess impressive speed, their long term use at temperatures of about 1300.degree. C. again resulted in a significant percentages of failures.
Therefore, there is a need for a aluminum nitride-based support zone which does not alter the electrical characteristics of the igniter, does not develop oxidation problems during use, and does not pose densification nor machining problems during manufacture. In particular, there is a need for a support zone which solves these problems for the igniter disclosed in Willkens '565.